1. Field of the Invention
The invention relates to porous plastic sheeted and shaped materials which find utility in a variety of applications, including use as separators, membranes, filters and diaphragms. The invention also relates to a method for the manufacture of such materials.
2. Description of the Prior Art
The addition of a water-insoluble solid additive of a preselected grain size to polymer latices, for the purpose of forming a porous product, is well known in the art. For example, U.S. Pat. No. 2,542,527 discloses a process for producing porous PVC by incorporating water-insoluble particulate starch into the polymer. The starch particles are first swelled and then removed by immersion in an acidic solution. U.S. Pat. No. 2,997,448 discloses a method for producing a porous PTFE by the addition of plaster of Paris to an aqueous dispersion of PTFE. The particulate plaster of Paris is removed from the formed polymer by dissolution in an organic acid. British Pat. No. 1,081,046 also adds a water-insoluble salt to an aqueous dispersion of PTFE, which salt is subsequently removed from the formed product by hydrochloric acid or other solvent. Similar disclosures are made by U.S. Pat. Nos. 3,702,267, 3,890,417 and 3,930,979. While such processes produce a satisfactory porous and microporous products, they also present the problems of cost and handling of acids and other noxious solvents and of waste disposal of the spent solvent.
The prior art also discloses a number of processes for producing porous sheet materials by extraction of a water-soluble filler. Typical is the teaching of U.S. Pat. No. 2,846,727 which discloses a process involving the addition of a water soluble salt to an aqueous dispersion of ACRN. The major problem with such a process is that in the dissolution and recrystallization of the salt which occurs prior to its removal by leaching, it is difficult to control the particle size of the salt or the pore size of the final product.
It is also known to dry-blend various polymers with solid additives of a preselected grain size. For example, U.S. Pat. No. 3,943,006 discloses a process for producing a porous PTFE by dry-mixing PTFE powder with sugar or other crystalline substance. The problem with these processes is that it is difficult to obtain a product of uniform porosity.